A high-performance photovoltaic system usually comprises multiple strings that contain multiple solar modules each. In such a system, the solar modules of each string are connected to each other either in series or in parallel to ensure that the desired output voltage and the desired output current are present at each string. To connect multiple strings of this type of photovoltaic system to a central inverter that supplies electrical power from the photovoltaic system to an AC power grid, it is often the case that long power cables have to be installed whose length is determined by the surface area of each string as well as the spatial arrangement of all the strings. These power cables are a significant cost factor given their large cross-sections and moreover suffer from continuous power loss due to the electrical resistance they cause. In general, it would be possible to reduce the requisite cross-section of the power cables and the amount of power loss if the output voltage of each string is increased through the serial connection of more solar modules while the same number of solar modules with less parallel connections would reduce the power accordingly. However, this concept cannot be implemented in all cases as it would cause the output voltages of the strings to rise to dangerous levels. In the USA, for example, voltages exceeding 600 V to ground are generally prohibited in conventional electrical systems. In practice, this limits not only the output voltages of the strings in a photovoltaic system but also the possibility of conducting smaller currents at higher voltages so as to minimize the cross-section of the power cables.
Known system features implemented in the “Solaron Remote PV Tie (RPT)” by Advanced Energy Industries, Inc., Fort Collins, Colorado, USA (see www.advanced-energy.com). Based on these features, a string of a photovoltaic system with multiple solar modules connected in series and parallel is split into two serially connected substrings with a safety switch installed between them. On the output side, two power cables with a power switch installed in each are used to connect the string to an inverter that supplies electrical power from the photovoltaic system to a three-phase 480 V AC power grid. While the photovoltaic system is operating, the AC power grid also provides a ground reference point for the system. If the photovoltaic system is disconnected from the inverter and thus the AC power grid when the power switch in the power cables opens, the safety switch between the two substrings opens and the switches in the potential cables connecting the substrings to the inverter on both sides of the safety switch close. Disconnecting both substrings limits the maximum voltage generated at each point of the photovoltaic system and thus the maximum voltage to ground to 600 volts. Via the potential cable, the two substrings can also be checked for the presence of ground faults, provided, of course, the photovoltaic system is switched off. If the photovoltaic system is switched on again, the switches in the potential cables to the substrings reopen at the same time the safety switch between the substrings closes to interrupt the contact provided by the potential cables between the substrings and the ground due to the potential reference point of the photovoltaic system via the 480 V AC power grid. Based on this known circuit arrangement, any ground fault that occurs while the photovoltaic system is operating gives rise to major short-circuit currents that must be interrupted by triggering a common line circuit breaker or residual current breaker. Moreover, while operating the photovoltaic system with the known circuit arrangement, the accumulation of voltages greater than 600 V to ground normally cannot be prevented. One advantage, however, is that only two power cables are needed to connect the string that supplies an output voltage of 1200 V to the inverter. All other cables between the inverter and the strings are not used to conduct power and can therefore have a comparatively small cross-section.